Field of the Invention
The present invention is directed to an apparatus and process for determining the humidity of a high temperature gas sample. More particularly, the present invention is directed to a self contained and/or portable high temperature apparatus for determining the humidity of a hot gas sample without the need for an instrument quality oxygen source, distilled water source, ice water source or cleaning fluid. Gas samples with temperatures as high as, but not limited to 600.degree. F., can be measured.
Description of the Background Art
It is often necessary to determine the humidity of a heated air stream. For example, in ovens or dryers it is desirable to monitor the humidity level of the cooking or drying air to achieve optimum cooking or drying levels. Sensors for determining the relative humidity of a low temperature gas are well known, for example Dunmore type Lithium Chloride (LiCl) cells. however, the relative humidity measured by such simple instruments without regard for the temperature of the air is not a good indicator of the quantity of water in the gas sample. Other low temperature devices that can be used to calculate more temperature independent moist air properties, such as "wet bulb/dry bulb" thermometers, are limited to temperatures well below 300.degree. F.
More elaborate humidity sensors have been unsatisfactory for commercial use because they required specialized fluids such as instrument quality oxygen, distilled water or ice water which are not readily transportable and are best used in a controlled, i.e., laboratory, setting. An example of such a conventional humidity determining device may be found in U.S. Pat. No. 2,643,464 to Hadady, which requires a temperature equalizing bath. Other known devices are impractical in that they require additional cooling or cleaning fluids circulating therethrough.
More recently, there have been developed solid state moisture detectors which do not require cooling or cleaning fluids, or the use of instrument quality oxygen, distilled water or ice water. An example may be found in U.S. Pat. No. 4,503,707 to Rosa et al. which uses an aluminum oxide capacitive sensor. However, such solid state sensors do not have high temperature heat resistance capabilities. For example, the solid state thin film polymer type humidity/temperature sensor manufactured by the Rotronic Instrument Corporation can perform measurements in environments only up to 302.degree. F. Higher temperatures will damage the thin film layers.
It is known from U.S. Pat. No. 4,221,058 to Zagorzycki to cool a hot gas sample before measuring the humidity of the sample in a sampling chamber. However, Zagorzycki is a simple drier control which uses a wet bulb type sensing element which requires saturation of a cloth with a salt solution. This introduces the problems associated with the conventional art such as that of Hadady, i.e., the need for special liquids which are not easily transported and applied in a controlled manner in practical settings. Additionally, the sample chamber in Zagorzycki is under negative pressure, which raises the possibility of inaccurate measurements due to air leaks into the sample chamber. Moreover, Zagorzycki is inherently incapable of being used in mass balance calculations which require determining a humidity ratio, since it does not sense the absolute air pressure, a parameter which must be known in determining the humidity ratio.